The invention relates to an optical switching element having two substrates provided with a first drive electrode and a second transparent drive electrode, said substrates enclosing, within a sealing edge, a layer of an electro-optical material which is switchable between a plurality of optical states.
Such optical switching elements are used, for example in (video) camera shutters, diaphragms, grey filters, but also in, for example applications for illumination.
An optical switching element of such a type usually comprises a liquid crystalline material or a PDLC material (polymer dispersed liquid crystal material) between the two supporting plates. The mutual overlap of the drive electrodes on the two supporting plates determine the switching surface area (active area) which, dependent on the voltage on the drive electrodes, transmits a larger or smaller quantity of light.
Both when using a liquid crystalline material and an electro-optical material such as a PDLC, the distance between the substrates must be the same throughout the surface area because variations of thickness of the layer of electro-optical material cause variations of the optical properties, so that the operation of, for example a shutter made with such an optical element is affected. A phenomenon referred to as xe2x80x9csmearxe2x80x9d may occur. Moreover, locally unwanted lens actions occur at locally thicker or thinner parts. To keep the two supporting plates spaced apart at a constant mutual distance, spacer elements in the form of rod-shaped or ball-shaped xe2x80x9cspacersxe2x80x9d which are spread (uniformly or not uniformly) on the surface are generally used. The presence of these spacers is at the expense of the effective aperture and gives rise to an increased xe2x80x9cflarexe2x80x9d. Moreover, contaminations may be left at the location of the spacers, when the optical switching element is being filled with liquid crystalline material.
It is an object of the invention to provide an optical switching element in which one or more of the above-mentioned drawbacks are obviated as much as possible.
To this end, an optical switching element according to the invention is characterized in that at least one further edge enveloping a part of the layer of electro-optical material and having at least one inlet aperture and at least one outlet aperture is situated between the two substrates within the area bounded by the sealing edge, said enveloping edge having substantially the same thickness as the sealing edge.
xe2x80x9cSubstantially the same thicknessxe2x80x9d is herein understood to mean such a thickness that the two substrates are spaced apart at the same distance by the different edges; the real thickness may differ, for example, because the sealing edge is only situated between the substrates and the further edge is situated between electrodes provided on the substrates (with or without extra layers such as, for example color filters).
Since, as it were, the two edges now take over the function of the spacers, these spacers may be entirely or largely omitted, at least within the further enveloping edge, while a uniform thickness is nevertheless maintained. Moreover, the two edges may be manufactured in the same process step, for example by means of silk screening of epoxy glue. The first sealing edge now functions with one or more enveloping edges as a rigid support for the two substrates, while a constant distance between the two substrates is obtained. In a further embodiment, the area between the two edges, which is usually unimportant or less important for the optical function of the optical component, is provided with spacer elements, which enhances the rigidity.
By providing the further enveloping edge with (preferably facing) inlet and outlet apertures, the electro-optical material, which is introduced in its liquid phase into the space within this further edge and is subsequently polymerized, if necessary, can flow on as far as a space between the sealing edge and the further edge. It is thereby achieved that, for example, contaminations in the liquid front (for example, dust particles which are taken up from an orienting layer or particles which are released by demixing the filling material (due to chromatography)), are deposited outside the xe2x80x9cactive areaxe2x80x9d (the area bounded by the further edge) and thus cannot detrimentally influence the transparency of the optical element in the light-transmissive state. To enforce the flow and prevent a possible flow, along the outer side, outside the active area, a further embodiment is characterized in that parts associated with the further enveloping edge between the inlet aperture and the outlet aperture extend as far as the sealing edge.
When a liquid crystalline material is used as an electro-optical material, a twisted nematic material between crossed polarizers is generally used, so that it is possible to switch between a substantially light-transmissive state and a substantially opaque state. The use of polarizers is, however, at the expense of absorption in the polarizers of a part of the light in the light-transmissive state (50% or more).
Therefore, an electro-optical material which is switchable between a light-transmissive state (50% or more) and a light-diffusing state is preferably used. Examples are PDLC material (Polymer Dispersed Liquid Crystal), NAPC material (Nematic Aligned Polymer Crystal), and (nematic) anisotropic gels with or without chiral additions. In the light-transmissive state, much more than 50% of the light is then transmitted. However, light is also transmitted in the diffusing state; in, for example, digital (video) cameras provided with one or more CCD sensors (Charge Coupled Devices), this causes a constant background noise which can be compensated electronically.
A further preferred embodiment of an optical switching element according to the invention is characterized in that the first and the second transparent drive electrode are provided with drive means and are structured in such a way that facing parts of the transparent drive electrodes are substantially completely bounded by the further enveloping edge. With the aid of the drive means, the active area may be switched from, for example, the light-transmissive state to the light-diffusing state, but intermediate states are alternatively possible.
The electrodes do not need to be formed as an integral part on the two supporting plates, but may be subdivided into sub-electrodes within the electrode parts bounded by the further enveloping edge, the separate sub-electrodes being provided with separate drive means. The active area is then subdivided into separately switching areas, for example annular areas with which a diaphragm function can be realized.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.